1. Field of the Invention
This invention relates to methods for preparing new crystalline titanium molecular sieve zeolite compositions.
2. Background of the Invention and Prior Art
Since the discovery by Milton and coworkers (U.S. Pat. Nos. 2,882,243 and 2,882,244) in the late 1950's that aluminosilicate systems could be induced to form uniformly porous, internally charged crystals, analogous to molecular sieve zeolites found in nature, the properties of synthetic aluminosilicate zeolite molecular sieves have formed the basis of numerous commercially important catalytic, adsorptive and ion-exchange applications. This high degree of utility is the result of a unique combination of high surface area and uniform porosity dictated by the "framework" structure of the zeolite crystals coupled with the electrostatically charged sites induced by tetrahedrally coordinated Al.sup.+3. Thus, a large number of "active" charged sites are readily accessible to molecules of the proper size and geometry for adsorptive or catalytic interactions. Further, since charge compensating cations are electrostatically and not covalently bound to the aluminosilicate framework, they are generally base exchangeable for other cations with different inherent properties. This offers wide latitude for modification of active sites whereby specific adsorbents and catalysts can be tailormade for a given utility.
In the publication "Zeolite Molecular Sieves", Chapter 2, 1974, D. W. Breck hypothesized that perhaps 1,000 aluminosilicate zeolite framework structures are theoretically possible, but to date only approximately 150 have been identified. While compositional nuances have been described in publications such as U.S. Pat. Nos. 4,524,055, 4,603,040 and 4,606,899, totally new aluminosilicate framework structures are being discovered at a negligible rate.
With slow progress in the discovery of new aluminosilicate based molecular sieves, researchers have taken various approaches to replace aluminum or silicon in zeolite synthesis in the hope of generating either new zeolite-like framework structures or inducing the formation of qualitatively different active sites than are available in analogous aluminosilicate based materials.
It has been believed for a generation that phosphorus could be incorporated, to varying degrees, in a zeolite type aluminosilicate framework. In the more recent past (JACS 104 pp. 1146 (1982); Proceedings of the 7th International Zeolite Conference, pp. 103-112, 1986) E. M. Flanigan and coworkers have demonstrated the preparation of pure aluminophosphate based molecular sieves of a wide variety of structures. However, the site inducing Al.sup.+3 is essentially neutralized by the P.sup.+5, imparting a +1 charge to the framework. Thus, while a new class of "molecular sieves" was created, they are not zeolites in the fundamental sense since they lack "active" charged sites.
Realizing this inherent utility limiting deficiency, for the past few years the research community has emphasized the synthesis of mixed aluminosilicate-metal oxide and mixed aluminophosphate-metal oxide framework systems. While this approach to overcoming the slow progress in aluminosilicate zeolite synthesis has generated approximately 200 new compositions, all of them suffer either from the site removing effect of incorporated P.sup.+5 or the site diluting effect of incorporating effectively neutral tetrahedral +4 metal into an aluminosilicate framework. As a result, extensive research in the research community has failed to demonstrate significant utility for any of these materials.
A series of zeolite-like "framework" silicates have been synthesized, some of which have larger uniform pores than are observed for aluminosilicate zeolites. (W. M. Meier, Proceedings of the 7th International Zeolite Conference, pp. 13-22 (1986).) While this particular synthesis approach produces materials which, by definition, totally lack active, charged sites, back implementation after synthesis would not appear out of the question although little work appears in the open literature on this topic.
Another and most straightforward means of potentially generating new structures or qualitatively different sites than those induced by aluminum would be the direct substitution of some charge inducing species for aluminum in a zeolite-like structure. To date the most notably successful example of this approach appears to be boron in the case of ZSM-5 analogs, although iron has also been claimed in similar materials. (EPA 68,796 (1983), Taramasso et al; Proceedings of the 5th International Zeolite Conference; pp. 40-48 (1980)); J. W. Ball et al; Proceedings of the 7th International Zeolite Conference; pp. 137-144 (1986); U.S. Pat. No. 4,280,305 to Kouenhowen et al. Unfortunately, the low levels of incorporation of the species substituting for aluminum usually leaves doubt if the species are occluded or framework incorporated.
In 1967, Young in U.S. Pat. No. 3,329,481 reported that the synthesis of charge bearing (exchangeable) titaniumsilicates under conditions similar to aluminosilicate zeolite formation was possible if the titanium was present as a "critical reagent" +III peroxo species. While these materials were called "titanium zeolites" no evidence was presented beyond some questionable X-ray diffraction (XRD) patterns and his claim has generally been dismissed by the zeolite research community. (D. W. Breck, Zeolite Molecular Sieves, p. 322 (1974); R. M. Barrer, Hydrothermal Chemistry of Zeolites, p. 293 (1982); G. Perego et al, Proceedings of 7th International Zeolite Conference, p. 129 (1986).) For all but one end member of this series of materials (denoted TS materials), the presented XRD patterns indicate phases too dense to be molecular sieves. In the case of the one questionable end member (denoted TS-26), the XRD pattern might possible be interpreted as a small pored zeolite, although without additional supporting evidence, it appears extremely questionable.
A naturally occurring alkaline titanosilicate identified as "Zorite" was discovered in trace quantities on the Siberian Tundra in 1972 (A. N. Mer'kov et al; Zapiski Vses Mineralog. Obshch., pages 54-62 (1973)). The published XRD pattern was challenged and a proposed structure reported in a later article entitled "The OD Structure of Zorite", Sandomirskii et al, Sov. Phys. Crystallogr. 24 (6), Nov-Dec 1979, pages 686-693.
No further reports on "titanium zeolites" appeared in the open literature until 1983 when trace levels of tetrahedral Ti(IV) were reported in a ZSM-5 analog. (M. Taramasso et al; U.S. Pat. No. 4,410,501 (1983); G. Perego et al; Proceedings of the 7th International Zeolite Conference; p. 129 (1986).) A similar claim appeared from researchers in mid-1985 (EPA 132,550 (1985).) More recently, the research community reported mixed aluminosilicate-titanium(IV) (EPA 179,876 (1985); EPA 181,884 (1985) structures which, along with TAPO (EPA 121,232 (1985) systems, appear to have no possibility of active titanium sites. As such, their utility is highly questionable.
That charge bearing, exchangeable titanium silicates are possible is inferred not only from the existence of exchangeable alkali titanates and the early work disclosed in U.S. Pat. No. 3,329,481 on ill defined titaniumsilicates but also from the observation (S. M. Kuznicki et al; J. Phys. Chem.; 84; pp. 535-537 (1980)) of TiO.sub.4 - units in some modified zeolites.